The present invention is directed to improvements in fiberizing techniques in which the fiberizing material in a molten state is applied to the exterior of the periphery of the fiberizing wheels, is carried by these wheels and is detached in the form of fibers by centrifugal force. These techniques make up a part of those referred to as "free" centrifugation, as opposed to the techniques in which the centrifugation element, in addition, plays the role of a bushing.
In these techniques, several fiberizing wheels placed adjacent one another are generally used. The material is passed from one wheel to another, each wheel transforming a part of the material received into fiber, and discharging the excess onto the next wheel.
The production of fibers by centrifugation is not limited to these techniques. It is also well known to direct the fiberizing material onto the front of a disk or a rotating spinner, or even to use a drum of which the periphery is perforated with a multitude of orifices forming a bushing from which the material escapes under the effect of centrifugal force.
These last two types of processes are distinguishable from those to which the invention pertains, both by the means employed and by the results which they attain. For example, the centrifugation by means of a disk in practice does not lead to adequately fine fibers for numerous applications; furthermore, the use of certain materials traditional in this kind of manufacture is impossible in a drum forming a bushing, due to, for example, an overly-elevated temperature treatment or the presence of defects in homogeneity of the material.
The techniques wherein fiberizing is carried out by placing the material on the exterior of the peripheral surface in the manner mentioned above, present certain advantages. However, in certain respects this type of fiberizing does not give total satisfaction, despite the numerous improvements which have been proposed up to now.
A first object of the present invention is to permit the implementation of this type of fiberizing under more satisfactory conditions. In particular, the invention proposes to improve the process and equipment for the formation and treatment of the fibers.
Another object of the invention is to improve the quality of the products prepared, particularly with respect to the treatment of the fibers.
Another object of the invention is to increase the fiber yield, that is, the percentage of the material recovered in the form of fibers in relation to the total material used, it being known that one of the primary disadvantages of this kind of technique is the presence of a relatively large quantity of unfiberized products.
Another object of the invention is to improve the quality of the products prepared, particularly the mechanical and insulating properties.
Another object of the invention is to facilitate the continuous operation of the installations by reducing the frequency and the duration of the stoppages required for periodic reconditioning of the equipment.
Due to the numerous factors which effect fiber formation and the finished product, a systematic analysis of these techniques is difficult. This partly explains the diversity of the proposals previously suggested for improving these techniques.
The first studies of these systems were directed toward determining the best conditions for centrifugation: introduction method of the fiberizing material, shape of the surface of the wheels, rotation speed, dimensions, relative position and number of wheels, and related factors.
Subsequently, the emphasis of the studies was placed on the conditions under which the fibers, after detachment from the fiberizing wheels, are treated and transported to the fiber collecting means.
Thus, it was found desirable to subject the fibers to a gas current at the time of their formation, which entrains and separates them from the unfiberized particles.
To improve these techniques, various proposals were also made concerning the treatment of the fibers for their use in the form of insulation mat. Generally, for these uses, the fibers are coated by spraying of a liquid binder composition, intended after drying and/or thermal treatment to confer its cohesion and its mechanical properties to the finished product. To be satisfactory, this treatment obviously must not disturb the so-called fiberizing operation. It must also lead to the coating of the fibers as uniformly as possible.
In one conventional form of treatment, the spraying of the liquid binder composition is carried out in the path of the gas current carrying the fibers toward a collecting surface, the spraying taking place at a distance from the fiberizing wheels. This manner of binder application does not result in a good homogeneity of the coating. The exact reason is unknown but it is noted that the flow of the gas current presents the fibers in an irregular manner to the spraying elements. Furthermore, it is all the more difficult to uniformly coat all the fibers because they have a tendency to become gathered in bundles or roves during their progression toward the collecting surface.
In order to try to improve the application of the coating, it has been proposed to project the sprayed composition into the path of the fibers on the other side of the fiberizing wheels with the same speed and in the same direction as those of the gas current carrying the fibers. However, such measures were not able to satisfactorily improve the results.
To avoid the aforementioned difficulties, it has been conventional practice to spray the binder composition in the vicinity of the fiberizing wheels into the gas current intended to carry the fibers and separate them from the unfiberized particles. In this case, the spraying is effected into these gases before they come in contact with the fibers. By operating in this manner, the fibers are brought in contact with the treating composition immediately upon their formation, under better controlled conditions. Nevertheless, this mode of coating involves certain disadvantages.
One of these disadvantages arises from the presence of the spraying elements close to the surface of the wheel. In this position, these elements are exposed to the heat radiating from the surface brought to high temperature by contact with the molten material. They are also exposed to the accidental projections of molten material. These spray elements especially have a tendency to retain the escaped fibers from the current which leads them toward the receiving surface. These fibers coated with composition, usually sensitive to heat, then undergo a "baking". Deposits are thus formed on the spray elements, rapidly resulting in the obstruction of these elements.
To minimize these disadvantages, spray nozzles can be used in which the orifices are relatively large, which for a given flow leads to reducing the number of nozzles. These nozzles, less numerous, are spaced further apart from each other and the result is a less homogeneous treatment, detrimental to the quality of the finished product.
Furthermore, regardless of the precautions taken, the progressive clogging is not completely prevented, which leads to the obstruction of the spray nozzles, and the interruptions for reconditioning cause relatively frequent stoppages of production.
In the apparatus containing several fiberizing wheels, from the point of view of the circulation of the material from one wheel to the other, it is preferable to avoid the blowing and/or the spraying between the wheels, particularly so as not to cool the material not yet fiberized. In general, an effort is made to form the fibers outside of these zones between the wheels. However, a certain quantity of fibers is either formed in or carried into these zones and is thus not directly touched by the sprayed composition, which increases the risk of defects in homogeneity of the product.